Polishing of aluminum and aluminum base alloys



United States Patent Ofitice POLISHING F ALUMINUM AND ALUMINUM I BASE ALLOYS Karl Guggenberger, Nurnberg, Germany, assignor, by mesne assignments, to Fromson Orban Co., Inc., New York, N. Y., a corporation of Delaware No Drawing. Application June 18, 1953, Serial No. 363,927

Claims priority, application Germany May 12, 1953 7 Claims. (Cl. 204-1405) The invention relates to a process for shining or polishing aluminum and aluminum base alloys by anodic treatment of the articles in suitable electrolytes. Various compositions have been proposed for such polishing baths, for example solutions of glacial acetic acid in water or solutions which contain fluoborate. Also, electrolytes which contain chromic acid and sulphuric acid have been proposed as polishing baths. However, it has hitherto been impossible to introduce these baths into practice, because the great wearing away of the material at the anode gives a rapid concentration of aluminum in the bath, and also a purely chemical attack of the articles by the electrolytes takes place and the chromic acid is progressively reduced by the hydrogen evolved at the cathode.

Exhaustive experiments have shown that it is possible to obviate these disadvantages, which have hitherto stood in the way of employing, in practice, electrolytes with sulphuric acid and chromic acid, so that it is possible to obtain excellent results, by a definite determination of the quantities, in the bath, of the two substances mentioned.

The process according to the invention is characterised by the feature that the electrolyte contains additions of chromic acid of, for example, 3 to 40 grams, or up to the saturation limit, with a content of 600 to 1750 gms. of sulphuric acid per litre. The best results are obtained with a content, in the electrolyte, of to 30 gms. of chromic acid and 1300 to 1600 gms. of sulphuric acid (H2804) per litre. However, the bath still gives a definite polishing effect when the content of chromic acid drops below 3 during working.

The temperature of the electrolytes should be 50 to 100 0., preferably 80 to 90 C. High current densities may be employed, as a rule from 25 to 50 amps. per square decimetre, but the current density may be increased up to 80 amps. per square decimetre. The voltage may be from 8 to 24 volts. Preferably, voltages of from 13 to 20 volts are worked with.

The period of exposure depends upon the nature of the surface of the parts being polished and the degree of purity of the material. It may be between 5 seconds and 6 minutes, according to the roughness of the surface. An example of the composition of the bath is 12 gms. chromic acid and 1550 gms. H2804 per litre.

In many cases, a further improvement of the economical working of the polishing baths with an almost constant effect can, according to the invention, be obtained by using, for carrying out the polishing process, cathodes which are inserted into a liquid-tight, acid-proof diaphragm. In this way, a powerful stirring up of the bath, which has a disturbing action on the polishing effect, is prevented by the cathodically evolved hydrogen. Any osmosis phenomena can be prevented by a suitable choice of the concentration of electrolyte in the anode space and cathode space. The diaphragms may be formed of separate cells which surround the cathodes. Alternatively, one or more diaphragm walls may be inserted, in a liquidtight manner, in the bath vessel, which walls divide the latter into a number of compartments, the largest of which forms the anode space which serves for the treatment of: the goods.

The process can be employed with very great success for polishing very pure aluminum containing 99.99% of' Al and commercial pure aluminum, for example alumi-- num containing 99.5 to 99.85% of Al. Alloys containing; a low content of magnesium (less than 3%) and alloys with a low content of copper and silicon can also be treated? with success.

Even in the case of pressed parts containing 3.8% Cu;- 0.6% Mg, 0.5% Fe, 0.75% Mn and 0.25% Zn, which it has not been possible to treat with success by chemical polishing processes or the already known electrolytic proc-- esses, perfect polishes are obtained by the process accord ing to the invention. This is also the case with Wire Ina-- terial containing 2.7% Cu and 2.18% Zn or alloys of. the Al-Mg-Si type with an addition of zinc.

The following is a detailed description of an example of carrying out the process according to the invention: The articles are first preliminarily cleaned by a solvent, for example trichlorethylene, and electrolytically degreased. After rinsing, the articles are treated in an electrolytic polishing bath according to the present invention. For receiving the bath liquid, an iron tank with a thick coating of lead is advantageously employed. The heating of the bath may be effected by inserted bath heaters with lead sheaths or indirectly by gas, steam or electrical heating members. A known suction device for gases or vapours is fitted at the edge of the tank. The article rods, which are used for suspending the articles, are connected with the positive pole of a source of current. The cathodes consist of lead. The current is to be switched on as quickly as possible after the articles are suspended therein, in order to prevent a purely chemical attack of the articles.

After the exposure is completed, the articles are rinsed by immersion in running water. They can then be subjected to further treatment, for example to an anodic oxidation of their surfaces by one of the electrolytic proc esses known for this purpose.

What I claim is:

1. A process for shining articles made of aluminum; or aluminum base alloys, which comprises polishing the articles by anodically treating them in an electrolytic bath 3 components of which consist per liter bath off the acid from 600 to 1750 grams of sulphuric acid and from 3; grams of chromic acid up to the saturation limit.

2. A process as claimed in claim 1, wherein said acidlf components consist, per litre bath, of from 1200 to 1600+- grams of sulphuric acid and from 3 to 40 grams of chromic; acid.

3. A process as claimed in claim 1, wherein said acidi components consist per liter bath, of from 1200 to 1600. grams of sulphuric acid and from 20 to 30 grams of chromic acid.

4. A process as claimed in claim 1, wherein said acidv components consist, per liter, of about 1450 grams of sulphuric acid and about 25 grams of chromic acid.

5. A process as claimed in claim 1, wherein said articles: are treated at a temperature from between 50 to 100 C.. and with a current density of from 13 to amperes per: square decimeter and a voltage of from 8 to 24 volts.

6. A process for shining articles made of aluminum or aluminum base alloys which comprises polishing the articles by subjecting them between 5 seconds and 6 minutes to an anodic treatment in an electrolytic bath the acid. components of which consist, per liter bath, of from 600* to 1750 grams of sulphuric acid and from 3 grams up to saturation limit of chromic acid at a bath temperature of 50 to C. with a current density of 15 to 80 am Patented June 19, 1956}.

peres persquare decimeter and a voltage between 8 and References Cited in the file of this patent 24 volts- UNITED STATES PATENTS 7. A process as claimed inrciaim 6, wherein a cathode 550 44 F A 24 1951 is used which is surrounded by a liquid-tight, acid proof aust diaphragm separating the catholyte from the remainder 5 FOREIGN PATENTS fthe bathllquld- 207,209 Switzerland Dec. 16, 1939 504,026 Great Britain Apr. 17, 1939 

1. A PROCESS FOR SHINING ARTICLES MADE OF ALUMINUM OR ALUMINUM BASE ALLOYS, WHICH COMPRISES POLISHING THE ARTICLES BYANODICALLY TREATING THEM IN AN ELECTROLYTIC BATH THE ACID COMPONENTS OF WHICH CONSIST PER LITER BATH OF FROM 600 TO 1750 GRAMS OF SULPHURIC ACID AND FORM 3 GRAMS OF CHROMIC ACID UP TO THE SATURATION LIMIT. 